The invention concerns a microphone array which comprises a multiple of microphones which are arranged in an elongated element or housing. The individual microphones in the microphone array are arranged in pairs, in that the individual microphones in each pair are placed on each their sides of a centreline for the microphone array, and in that the signals from the microphones are summated to form an output signal for the microphone array.
Microphone arrays of this type, which use direct summation of the signals from a finite number of microphones, display a directivity which is dependent on the frequency. The directivity generally depends on the effective length of the array and the acoustic wavelength at the relevant frequency. There is thus achieved only a minor degree of directivity at low frequencies (i.e. at frequencies where the wavelength L is much greater than the length of the array), and the directivity increases with the frequency until there is achieved a very high degree of directivity at wavelengths which are much shorter than the length of the array.
The lowest wavelength at which the microphone array can provide a certain degree of directivity is dependent on the overall length of the array, and the highest frequency at which the directional characteristic does not have significant side lobes is dependent on the distance between the microphones in the array.
The length of the array and the distance between the microphones (and herewith the number of microphones) thus depends on the frequency range in which a given directivity is desired within certain limits.
Such microphone arrays which are configured with the object of achieving a good directivity are used, for example, in connection with conferences and meetings, where a microphone is positioned to detect the sound from one or possibly more speakers, but not from speakers who are situated in another part of the room and who possibly use other microphones. Moreover, such microphone arrays are used in connection with teleconferences, video-conferences and the like where it is similarly desired to detect sounds from a speaking person without also picking up disturbing noise from other persons or background noise in general.
A special use will be in connection with personal computers and the like, where it can be envisaged that a microphone array can be placed in the vicinity of the screen, for example on top of it, so that speech from the user of the screen is detected by the microphone.
It is important for such applications that the microphone array is small in extent, so that it can easily be placed in an expedient position, and that it is of a reasonable price, which among other things means that it needs to be relatively simple in its configuration without containing too many and too complex components.
Microphone arrays of the kind defined in the introduction are known, for example, from U.S. Pat. No. 4,311,874, where use is made of a relatively large number of microphones in each microphone array in order to achieve the desired degree of directivity. The microphones in this array are arranged in such a manner that the distances between the microphones are not the same, i.e. not equidistant.
Furthermore, microphone arrays are known where the microphones are arranged at varying distances, and where the microphones are connected to different kinds of filters. This is known for example from DE publication No. 36 33 991, where use is made of bandpass filters with frequency bands which are adjacent to each other.
The object of the invention is to provide a microphone array which with relatively short length, with a relatively small number of microphones and relatively simple means, can display a high degree of directivity.
This is achieved with a microphone array which is configured as disclosed in claim 1. By filtering the microphone signals so that microphones, depending on their distance to the centre plane, are not active for higher frequencies, it is achieved that the effective length of the array can be held proportional to the wavelength over a certain frequency range, so that the directivity can be held constant over the relevant frequency range. Moreover, it is achieved that with a suitable choice of the precise positions of the microphones, and a correspondingly suitable choice of filter characteristics, the directivity can be determined depending on the frequency over a wide range, while at the same time the number of microphones is held at a suitably low level.
With an expedient embodiment as disclosed in claim 2, it is achieved that the microphone array has a constant directivity, i.e. independent of the frequency, up to an upper frequency f0 with the use of a minimum number of microphones and with a given length of the array. The constant directivity is achieved from the frequency f0 down to the frequency f0/3. Moreover, it is achieved that the directivity is the highest possible in a frequency range from f0/3 down to f0/10. By using unidirectional microphones e.g. unidirectional 1. order gradient microphones, it is further achieved that the main lobe of the microphone array is associated with only one side of the array.
With the especially expedient embodiment as disclosed in claim 3, there is achieved a microphone array which has constantly high directivity in the range from 5000 Hz down to approx. 1670 Hz, and which furthermore has the highest possible degree of directivity from here and down to approx. 500 Hz, i.e. in an area in which a large part of the frequency range for human speech lies.
With another embodiment such as that disclosed in claim 6 and 7, there is achieved the further advantage for the user that it can be immediately ascertained whether the person concerned is situated in the area for the main lobe, which is very important when using microphone arrays with a high degree of directivity.